1. Field of the Invention
The invention described herein is directed to an apparatus over which sheet material is placed to effect tensioning or stretching thereof. More specifically, the inventive concept is directed to such an apparatus that supports the sheet material in a manner that minimizes damage thereto when the sheet material is under tension.
2. Description of the Prior Art
Sheet material tensioning is practiced in a wide variety of applications. Various fabric stretcher frames have been designed to produce tension in sheet material in such fields as cargo hold coverings, silk screen printing and artist's painting canvases. Such stretcher frames generally consist of interconnected frame sections that are separated at portions of their interconnection points to expand the frame. However, as will be described in the paragraphs that follow, such expansion exposes the fabric being tensioned to planar discontinuities defined by abrupt edges that can cause deformities and weakness therein.
Referring to FIG. 1, there is shown a canvas stretcher frame typical in the prior art. The stretcher frame includes a plurality of frame sections, such as those shown at 102 and 104, interconnected at a tongue and groove mitre joint, generally shown at 110. The frame sections 102, 104 are fitted together and a pair of wedges 106, 108, sometimes referred to as “keys”, are inserted into slots 112, 114, respectively, formed in the ends of the frame sections 104, 102 at the joint 110. When the frame has been assembled, a sheet of canvas (not shown) is affixed to the frame and is supported by a material-supporting lip 116 formed on the frame members 102, 104. Tension is then applied to the fabric by driving wedges 106, 108 deeper into their respective slots 112, 114. The wedging force applied by the wedges 106, 108 induces a separation between the frame members 102, 104 at the joint 110. In so doing, the planar continuity of the material-supporting lip 116 is broken to define a sharp corner, representatively shown at 118, at the joint 110. Under tension, the discontinuity of the material-supporting lip 116 introduces high stress points in the material that can produce a weakness therein, which, over time, is likely to propagate to other areas of the material. This is particularly undesirable in canvas painting surfaces where great care is taken to prepare the canvas so that the artwork disposed thereon will last for generations. Such weaknesses caused by uneven tensioning and warping of the stretched fabric are known to cause cracks in the medium, eventually necessitating usually expensive repair work.
Another stretcher frame of the prior art is illustrated in FIGS. 2A-2B, which is described in detail in U.S. Pat. No. 3,625,274. The frame includes a plurality of tubular frame sections, such as those shown at 202 and 204 interconnected by a corner insert 206. An expander element 208 is provided at the corner of the frame and an adjustment screw is threaded into the expander element 208. The adjustment screw 210 engages with an inside corner of the corner insert 206. As the screw 210 is rotated, edges of the expander element 208 engage with the ends of the frame members 202, 204 to impose a greater separation between those elements. Whereas this type of stretcher frame avoids a gap at the corner, it still exposes the fabric (not shown) to planar discontinuities in the form of abrupt edges, such as that shown at 220 in FIG. 2B. Again, the discontinuous fabric-supporting edge introduces a deformity and a possible weakness in the material, which can become worse over time.
Given the shortcomings of the prior art, the need is apparent for a sheet material tensioning apparatus that avoids such planar discontinuities in the edges of the frame.